The polymerization of lactams in molds has been used and studied for several decades. The latest efforts in improving the technique of polymerization of lactams have been focused on modifying the catalytic system (composed by activator and initiator) to carry out the polymerization in a controlled and selective manner. However, the way in which the basic ingredients are mixed and fed to the mold has not changed and is still basically one of the following two forms: In the first form, two independent pre-mixtures are prepared, the first one comprises a mixture of the activator and a fraction of the lactam; and the second one consists of the initiator and the remaining fraction of the lactam. Both pre-mixtures are subsequently combined to carry out the polymerization.
The previous techniques allow producing serial parts of a composition, but they are not suitable in the continuous manufacture of non-serial parts, for example with individual sizes and/or geometries, different compositions or characteristics.
European patent application EP2338665A1 describes a process and device for polymerizing lactams in molds, wherein the lactam, activator and initiator are independently fed and dosed to a mixing head feeding a mold. The described process for polymerizing lactams can be catalyzed by a great deal of initiators and activators, some of them being solid. Solid initiators and activators must be melted and kept at high temperatures during the whole process. The described device for molding lactams is versatile for a great deal of initiators and activators. Such versatility makes the use of the device difficult, since heating means are required in each and every dosing elements of the circuit through which each of those materials (initiators or activators) passes. The three components (lactam, activator and initiator) of the polymerization process are dosed separately. This leads to two of the advantages of the described process: (1) stability in time is assured and (2) different dosing (in %) of each component is permitted for each type of pieces or parts to be manufactured.
However, it has been observed that melting and dosing the activator and the initiator at high temperatures is difficult. Besides, both materials must be added to the mix in very specific percentages, which complicates their correct dosing by means of conventional gear pumps. Furthermore, since solid initiators and activators must be melted at higher temperatures than room temperature, and their melted state presents viscosities which are dependent on temperature, their temperature must be kept constant and precise during the whole process of dosing.
The different elements of the device described in EP2338665A1 are each heated independently from each other, by means of individual heating systems, either electric systems based on resistances or by means of thermic fluids. However, individual heating of those elements cause a cold area in the connection element (by valves, nuts or any other connection elements) between each pair of elements. For example, the caprolactam becomes solid at a temperature lower than 70° C., and these cold points can produce cooling and subsequent solidification of the monomer, thus producing blockage in the dosing systems.
In sum, a device is needed, which enables to solve in an efficient way, the above mentioned problems of conventional processes and devices for polymerizing lactams in molds.